The perfect planting site would be one that is sunny, has good drainage and is free of weeds or choking grasses.  Most areas have one or maybe two of these aspects but some work will be required to get all three.  If your area has some shady areas, you can choose flower selections that do well in partial sun and a few will survive in shade.  Good drainage can be achieved with some shovel work and soil amendments suggested by your local Natural Resources Conservation office or Agriculture extension office.  To optimize your expenses and efforts now and in the future it is very important to eliminate unwanted weeds and choking grasses that can and will out-compete your new wildflowers.
Everybody wants to know, “Where did all these weeds come from?” What is the origin of the weeds that appeared with the germinating wildflowers and grasses?  Were they in the seed mix?  Here is where we try to remind prospective planters that there are tons of weed seeds present in the top soil of any area and those that are deeper than germination level for that seed type will remain dormant down there under the soil for decades.  The majority of weed seed that is within germination level has been receiving the light and moisture necessary for germination and new seeds get put down every year. Sometimes there are multiple generations of weed seeds put down each year.   When the topsoil is tilled, all of those dormant weed seeds that were not a threat down deep are now going to germinate and grow.  One grower, trying to rejuvenate an old family garden, even removed 2 feet of topsoil in order to get rid of several years of weed seeds due to neglect.  The spring rains brought an amazing number of new weed sprouts from seeds that had been covered 2 feet deep.
You will need to loosen the soil before planting your new wildflower seeds or wildflower and grass mix seeds, however, avoid deep tilling.  Try to only disturb the top 2-3 inches of soil, then rake smooth.  Allow the weed seeds that have been exposed, to germinate, then hoe or hand pull them.  Leave them to dry out on the surface of the soil, to become an amendment for your soil.  Do not allow any weeds to seed out, pull or destroy all before they go to seed.  If you have time, repeat the process, by allowing another round of weeds to sprout up for subsequent destruction before planting your wildflower seed bed.  If you keep on top of the weed issue, you will eventually reduce the number of weed seeds that are left to germinate and your wildflowers will grow and fill in without competition.